Traditionally, remote access communication using the asynchronous mode of operation has relied on simple parity schemes on each byte of transmitted data, e.g., parity bits on byte boundaries, for error detection. This usually meant that one bit out of the 8 bits was reserved for parity and the remaining 7 bits were used for data. In general, however, all 8 bits in a byte may be required for use by higher layers in a communication hierarchy. Furthermore, the single-bit parity schemes enabled these existing systems to merely detect the error without provision for automatic correction and/or recovery. It was left to the end-user to attempt recovery by requesting re-transmission of data that looked garbled. With the advent of Videotex and concomitant presentation data encoding schemes, such as the North American Presentation Level Protocol Syntax (NAPLPS), the need for recovery from transmission errors became more apparent than hitherto foreseen. This is due to the high level of compression that these data encoding schemes utilize. An error in a single byte could lead to dramatic differences in what the end-user might perceive. The frequency and extent of such errors could seriously detract from the ease of use of Videotex.
The prior art has attempted to improve error control while avoiding loss of data capacity in asynchronous communications by stripping the parity bits and reformatting the data into packets with checksum bits for transmission, and then converting the received data back to usable standard form. An example of such a system is disclosed in U.S. Pat. No. 4,377,862 to Koford. However, such systems require special hardware to be compatible with standard modems and cannot be implemented purely with software. Other known attempts or possible attempts to deal with the problem, such as bisynchronous or other mode transmissions, would all appear to require special added hardware or different and more costly operating systems. Further prior art examples are found in U.S. Pat. No. 4,304,001 to Cope and U.S. Pat. No. 3,676,859 to Holloway et al.
In any event, no system is believed to presently be known which offers low-cost, high-efficiency, automatic detection and correction of asynchronous transmission errors and which is capable of use with standard modems. The system of the present invention is directed toward this end.